Permanent magnet rotor type ammeter



April 14, 1964 M J KRAMER 3,129,384

PERMANENT MAGNET ROTOR TYRE AMMETER RESPONSIVE TO THE FLUX OF A CONDUCTOR CARRYING CURRENT TO BE MEASURED Filed June 18, 1962 n 10 12 I3 15 I4 2 1 f PE 3" I a 1? o 7B 50 2 2O T. mmvroze. I J 3 Max J Kramer \q 1 BY ATTORNEY United States Patent Office 3,129,384 PERMANENT MAGNET ROTOR TYPE ANEMETER RESPGNSIVE TO THE FLUX OF A CONDUCTOR CARRYING CURRENT TO BE NEASURED Max J. Kramer, Par-ma, Ohio, assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania Filed June 18, 1962, Ser. No. 203,201 Claims. (Cl. 324-146) This invention relates to DC. ammeters and particularly to anode bar meters especially designed for measuring the anode bar currents of electrolytic aluminum producing cells or pots and the like.

An object of the invention is to provide a portable anode bar ammeter for use by an electrolytic pot-line operator to measure with good accuracy the anode currents of the electrolytic pots he is tending in a pot room so that the anodes may be set or positioned in the individual pots for uniform sharing of the load current and efiicient operation.

Another object of the invention is to provide an anode bar ammeter which will be of rugged construction to withstand rough usage and handling and be free from erratic readings of anode bar currents despite the presence of strong stray fields which are inherently present.

A further object is to provide an anode bar ammeter of generally simplified and improved form, simple to use and easy to read by unskilled workmen.

A practical embodiment of my invention is illustrated in the accompanying drawings, wherein:

FIG. 1 is a diagrammatic plan view of a multi-anode electrolytic cell or smelting pot with gas hood and collector omitted and showing a typical anode and cathode but arrangement for the cell, a large number of such cells being disposed in-line to constitute a pot line;

FIG. 2 is a plan view of the ammeter in current measuring position against an anode bar;

FIG. 3 is a longitudinal sectional view of the ammeter taken on the line III-HI of FIG. 2,

FIG. 4 is a cross-sectional view on the line lV-IV of FIG. 3, and

FIG. 5 is a cross-sectional detail view of the meter movement taken on the line VV of FIG. 3.

In the operation of electrolytic aluminum producing cells or pots in series in a pot-line, it is required to determine the amount of current each of the carbon anodes of a pot is carrying and to set the anodes so that each of them will share the load and carry about the same amount of current. Current measurement is presently a complicated technique achieved by reading the millivolts across a small length of the anode bar, and use of a conventional ammeter is not practical by reason of the high load current magnitude and the presence of strong stray fields, particularly near the load current supply areas of the respective pots, such as buses 16 and 17 hereafter described.

As is indicated in FIG. 1, a typical pot of a pot-line comprises a rectangular, carbon lined metal shell with embedded collector bars joined to a surrounding cathode bus 11 and usually a gas collecting cover or hood (not shown) over the pot and two rows of spaced carbon anodes 12 partially immersed in the molten electrolyte contained in the pot. The anodes are carried by vertical copper anode bars 13 extended through the hood and suspended from an overhead steel framework 14, a lower portion of which is supported from an upper fixed portion through adjusting screws (not shown), whereby the anodes may be raised and lowered as a group with respect to the molten metal level in the pot. The lower frame portion includes an anode clamp-on bus 15 to which the anode bars are individually clamped by the of the ammeter taken 3,129,384 Patented Apr. 14, 1964 customary releasable clamps (not shown). Thereby, the anodes are connected electrically in parallel to the anode bus of the pot and each anode is adapted by vertical adjustment to be set in operating position with respect to the molten metal level to take its share of the load current. Connecting bus bars 16 extending from the cathode bus 11 of each pot in the line and rising to the level of the anode bus 15 of the next pot and connected thereto through flexible strap conductors 17 serve to connect the pots electrically in series circuit relation, the D0. power supply being connected to the end pots in the line. Typically, in a ZO-anode pot operated at 60,000 amperes D.C., for example, the share of the load current by each anode should be about 3000 amperes.

In accordance with the invention, a direct reading ammeter is provided for measuring the anode bar currents by application of the meter to the face of each bar at an exposed point below the clamp-on bus and the top of the gas hood. This requires the instrument to be compact and of light weight, as well as rugged to withstand rough handling. In general, it comprises an encased rotatable magnet meter movement and top readable scale at one end of a handle for endwise positioning against an anode bar so as to be operatively influenced by the magnetic field around the bar due to the current flow therein. For minimum weight and structural simplicity and strength, I prefer to provide a fiat aluminum mounting plate 20 on which all the other parts are assembled, the upper portion of the plate being of sector shape and its lower portion formed with a pair of outwardly and forwardly projecting side wings 21. To these side wings, handle straps 22 are secured and to the straps a handle 23 of suitable length and material is secured so as to provide a forwardly extending handle for convenient applica tion of the instrument to the face of an anode bar.

Between the wings 21, a bore 24 (FIG. 3) is formed in the plate 20 with its center on the vertical center line 25 (FIG. 4) of the plate. A cover attaching flange 26 is provided on the front of the plate. For low cost, the flange is a thin aluminum strip bent into shape to surround the bore on three sides and to extend around the upper sector portion of the plate slightly inside the edge thereof. It is set edgewise against the face of the plate and cemented in place with epoxy cement. A conformably shaped cover box or casing 27, suitably formed of transparent plastic material, fits over flange 26 and is secured thereto by means of screws 28. If desired, .the cover box may be formed of other light weight material and provided with a window transparency in its arcuate top. Underlying the top of the cover box is a graduated scale 29 mounted on a holder 30 that is secured to the plate 20 and over which scale the inturned pointed end of a radial pointer 31 of the meter movement is adapted to move.

The meter movement comprises a non-magnetic probe in the form of a tube 32 (FIG. 3) which is snugly received in the bore 24 and is peripherally cemented to both faces of the plate with epoxy cement. It extends rearwardly from the plate in right angle relation thereto. The tube may be a piece of inch copper water tube of short length, between 1 and 3 inches being suitable, and is closed at its outer end by a copper cap 33 which is soldered thereto. Within the tube, a shaft 34 is coaxially journalled in spaced discs 35 and 36 of copper or other non-magnetic material which are mounted, as by a light press fit, in the cap 33 and in the tube, respectively, and serve as shock-resistant outer and inner bearings for the shaft. The discs are merely centrally bored to receive the shaft with a running fit. Between them, the shaft is slightly enlarged in diameter to provide shoulders which cooperate with the discs to limit shaft end-play. The shaft extends into the cover box 27 and the pointer 31 is secured thereto. magnetic material.

A cylindrical permanent magnet rotor or armature 37 of about /51 inch diameter is coaxially fixed on the enlarged portion of the shaft 34. It is made of high coercive force magnetic alloy material, preferably Alnico V for magnetic stability, and is permanently magnetized across a diameter thereof so as to have diametrically opposite north and south poles N and S, FIG. 5, its line of polarization or flux line being definite and fixed, as is well known in the art. Magnetically cooperating with the rotor are two essentially semi-circular magnetic stator pieces or poles 38 and 39 of uniform thickness which extend full length between the bearing discs 35-36. They encircle the rotor in concentric, radially spaced relation thereto so that an annular air gap 40 is provided therebetween and have their opposed longitudinal edges equally spaced apart to form narrow air gaps 41 diametrically opposite to each other in the magnetic circuit, hereinafter termed peripheral or restraining air gaps. The pole pieces are secured to the inner wall of the tube 32 and cementing them in position with epoxy cement represents a simple and adequate mode of securement. They may readily be produced as mates from inch O.D. seamless steel tubing of about inch wall thickness cut lengthwise along diametrically opposite lines and assembled in the tube 32 with their opposed edges equally spaced apart to form the restraining air gaps 41. It has been found that the width of these gaps should be between inch an /32 inch for satisfactory metering of the bar currents. If desired, they may be filled with non-magnetic metal, brazed or welded, to form an annularly solid stator sleeve radially insertable into the mounting tube.

In assembling the instrument, before the tube 32 with the stator poles mounted therein is secured to plate 20, it is turned on its axis to bring the air gaps 41 into alignment with the plate center line 25, so that the center line or line of symmetry A through the stator poles will be perpendicular to line 25 and in parallelism with a transverse plane across the plate through the tops of wings 21. The magnetized rotor 37 in its zero position always lines up its poles with the high reluctance points of the magnetic circuit, which are the air gaps 41 between the op posed edges of the stator poles. Accordingly, the N-S poles of the rotor 37 normally will be in perpendicular or 90 relation to the reference plane across the side wings 21. This orientation is important in order that the rotor and stator will be properly oriented in and linearly responsive to the annular magnetic field around an anode bar due to the bar current when the ammeter is held in upright current measuring position against the bar with the side wings 21, or the lower edge of the plate, in a substantially horizontal plane.

The rotor 37 is arbitrarily set in the probe with its north pole N facing the bottom air gap 41 and it rotates clockwise in operation to swing pointer 31 up-scale from its position at the left end of the scale. In zero position, since the air gaps 41 are narrow, the permanent magnet flux leaving the north pole of the rotor equally divides at the bottom air gap to flow through the stator poles, recombines at the top air gap and re-enters the rotor at its south pole S. The rotor is thus equally attracted to the opposed edges of the stator poles at the air gaps 41 so that its zero position is with its line of polarization lined up with the center of the air gaps 41 and is thus strongly magnetically biased and restrained to remain in zero position.

A weak torque spiral spring 42 is secured at ohe end to the shaft and at its other end to a pin 43 projecting from a copper zero adjusting disc 44 which is rotatably disposed in the open end of the tube 32. By rotational adjustment of disc 44, the spring normally exerts zero torque on the shaft for zero pointer position and it is held in adjustment against the bearing disc 36 by 21 Both are of nonretainer clip 45 secured by screws 46 to the front face of plate 20, thereby also retaining the rotor within tube 32 and permitting its removal, along with the two innermost discs, whenever desired. A clearance bore in disc 44 permits free passage of the shaft therethrough.

The spring serves other functions besides forcing the rotor back to its zero position. One purpose of the spring is to extend the linearity of the meter in this manner: as anode-bar current increases, the rotor 37 moves farther away from the stator air gaps 41, thus weakening the magnetic restraint. The torque-spring restraint is increasing since it is directly related to the rotor deflection, and the combination of the two restraints makes the deflection of the pointer linear with amperes and also extends the full scale range of the meter. A short mounting tube or probe 32 is illustrated and in case a longer one is used, a spacer sleeve is merely interposed between the discs 35 and 44 and a longer shaft provided. In all cases, the rotor and stator are located near the outer end of the probe so as to be closely adjacent to the face of an anode bar when in current measuring position.

To protect the soft tube 32 and cap 33 and the meter movement from mechanical damage, as when the instrument is impacted against an anode bar, or accidently dropped, a tubular non-ferrous bumper member 47 is provided. It surrounds in spaced relation to and projects slightly beyond the meter movement assembly and includes a flange plate 48 (FIG. 2) at its inner end by which it is secured by screws 49 to the back face of plate 20. Suitably, it may be made from non-magnetic stainless steel. The fiat outer end of the bumper member, being normal to the axis of the meter movement assembly, facilitates positioning the instrument against an anode bar in right angle relation thereto, instead of in an undesired tilted position.

In use, the ammeter is placed in a reasonably level position against an anode bar 13 with the side wings substantially horizontal. This disposes the stator poles 38 and 39 substantially in line with the magnetic field which is produced around the anode bar by the current flow through the bar and disposes the air gaps 41 and the flux line of the rotor substantially in relation to this field. With the current flow downward in the bar, the field is in the direction of the arrow B in FIG. 5 which polarizes the stator poles and causes field flux to flow from pole 38 across the annular and the restraining air gaps 40 and 41 to pole 39. This field flux reacts upon the magnetized rotor 37 to make it rotate in a clockwise direction against the reluctance restrain afforded by the air gaps 41 and the small counterforce of the spring 42 to an angular position generally related to the current strength in the anode bar. There are magnetic forces of attraction and repulsion developed on the rotor 37 at each pole gap 41 by which the rotor is caused to deflect or turn away from its zero position, while the air gap restraint does not disappear but diminishes with rotor deflection until the rotor turns about 25 from its zero position, whereat all further restraint is afforded by the spring 42. The extent of rotor deflection is indicated by the pointer on the calibrated scale.

It is inherent in the operation of reduction pots of a pot-line that stray magnetic field lines are present which affect the operation of magnetic meter movements. In the present case, however, the stator poles 38 and 39 have multiple functions which render the present meter movement quite effective and useful. They are made to encircle the rotor completely, except for the narrow air gaps 41 between their opposed edges. Thereby, they provide restraining torque on the magnetic rotor, since the north-south rotor poles are mutually attracted to the stator pole air gaps. They serve to weaken the strong field from the anode bar under measurement by distributing this field substantially uniformly to the rotor, thereby allowing the use of a weak restraining spring. They act effectively to shield the rotor from stray magnetic lines that are coming in at an angle in space much dilferent from the normal field of the bar under measurement by leading such stray field around the rotor, while the narrow width of the air gaps 41 reduces the amount of stray field that can directly reach the rotor to a For shielding effectiveness, the stator poles should remain magnetically unsaturated, yet be of minimum mass in order not to serve as a flux collector and amplifier, and making them from the steel tubing of the size and thickness heretofore mentioned has been found to make the meter movement effectively self-shielding. This shielding of the rotor, of course, increases with rotor deflection from its zero position. It is also found desirable to mount a thin gage iron strap 50 of a width substantially equal to the depth of the cover box 27 around the bottom and sides thereof, either inside or outside the same, to aiford additional shielding for the meter movement.

The ammeter of the present invention is a rugged, compact, low cost, direct reading operating tool for use by a pot tender who customarily has a given number of reduction pots of a pot-line to take care of, so that he may take current readings of the carbon anodes of each pot and set them, as needed, for sharing the load properly. As such a tool, the present construction has been found adequate in performance for pot loads up to about 80,000 amperes and has dispensed with the need for skilled supervisors to determine anode bar currents, usually by the complicated millivolt-drop and bar temperature mode.

Having thus described the invention and the manner of constructing and using the same, it will now be apparent to those skilled in the art that various changes in materials and details may be made without departing from the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. An ammeter for measuring anode bar current of electrolytic reduction cells and the like comprising,

(a) a mounting plate provided with a probe in the form of a tube projecting from its rear face and with a cover box on its front face and with an elongated handle extending forwardly of its front face for placement of the outer end of the probe against a vertical anode bar with the plate in upright position, both plate and tube being of non-magnetic material,

([2) a shafted cylindrical permanent magnet rotor polarized across a diameter and coaxially rotatably mounted within said tube in proximity to the outer end thereof and having its shaft extending into said cover box,

(0) a radial pointer on said shaft adapted to cooperate with a graduated scale on said plate, at least a portion of said box being transparent for scale visibility,

(d) a pair of essentially semi-circular, magnetic stator poles of uniform thickness secured to the inner wall of said tube in encircling relation to said rotor and separated by narrow air gaps at their opposed peripheral edges, whereby the rotor is magnetically held and restrained to remain in zero pointer scale indicating position by its own flux with its fiux line in alignment with the center of said air gaps,

(e) said stator poles having a fixed position with respect to a transverse plane across said plate which disposes said air gaps in perpendicular relation to and the center line of said poles in parallelism with said plane for providing a substantially corresponding relationship of said rotor and stator poles with respect to the annular magnetic field around the anode bar when the ammeter is in current measuring position against the bar,

(f) and a return spring secured between said shaft and said plate for lightly biasing said rotor to return to its zero position and producing scale linearity by compensating for loss of air-gap restraint with increasing current,

(g) said stator poles having a thickness of about inch for minimum mass and to remain magnetically unsaturated so as to continuously effectively shield said rotor from stray magnetic field lines inherently present at said cell, and the width of said air gaps being less than inch to minimize stray field lines passing therethrough to the rotor.

2. An electrolytic cell anode bar current ammeter comprising,

(a) a non-magnetic plate provided with a cover box and a forwardly extending handle on its front face and a rearwardly extending tubular probe on its rear face, and adapted for placement of the outer end of said probe against a vertical anode bar of said cell with said plate in upright position,

(b) said probe comprising a non-magnetic tube closed at its outer end and supporting a pair of bearing discs therein, one adjacent to said closed end and the other spaced therefrom,

(c) a shaft journalled in said discs coaxially of said tube and extending into said cover box,

(d) a radial pointer on said shaft having an inturned pointed free end movable over an arcuate graduated scale carried by said plate under the top of said box, at least the top of the box being transparent for scale visibility,

(e) a cylindrical permanent magnet rotor polarized across a diameter thereof fixed on said shaft between said discs,

(I) a pair of essentially semi-circular, magnetic stator poles of uniform thickness secured to the inner wall of said tube, extending full length between said discs in encircling, radially spaced relation to said rotor and having their opposed edges equally spaced apart to provide diametrically aligned, narrow restraining air gaps at opposite sides of said rotor, whereby the rotor is magnetically held and restrained to remain in a zero pointer scale indicating position by its own flux with its flux line in alignment with the center of said air gaps,

(g) said stator poles having a fixed position with respect to a transverse plane across said plate which disposes said air gaps in perpendicular relation to and the center line of said poles in parallelism with said plane for providing a substantially corresponding relationship of said rotor and stator poles with respect to the annular magnetic field around the anode bar when the ammeter is in current measuring position against the bar,

(h) and a return spring secured between said shaft and plate for lightly biasing said rotor to return to its zero position and producing linearity of rotor angle with load,

(i) said stator poles having a thickness of about inch for minimum mass and to remain magnetically unsaturated so as to continuously and effectively shield said rotor from stray magnetic field lines inherently present at said cell, and said air gaps having a Width of less than inch to minimize the stray field lines passing therethrough to the rotor.

3. An ammeter as defined in claim 2, wherein a tubular bumper member of non-magnetic metal is secured to the back face of said plate and surrounds said probe in radially spaced relation thereto, said member extending slightly beyond the outer end of said probe to protect the probe against mechanical damage, and having a flat outer end normal to the axis of the probe to facilitate positioning the ammeter against the anode bar in right angle relation thereto.

4. An ammeter as defined in claim 2, wherein said cover box includes a thin gage iron strap around the bottom and sides thereof, of a width substantially equal to measuring 7 the depth of the box, to aiford additional magnetic shielding for the meter movement.

5. An ammeter as defined in claim 3, wherein for lightness and economy of construction said plate is formed of aluminum and is provided with forwardly projecting side wings at the lower portion thereof to which said handle is secured, said plate having a bore through it centrally of said wings through which the inner end of said probe tube projects and is peripherally cemented to both faces of said plate to retain the tube in right angle relation to the plate, said tube comprising a length of inch copper water tube with a copper cap soldered to its outer end, and said stator poles comprising mates produced from inch O.D. seamless steel tubing.

No references cited.

UNITED STATES P A TENT OFFICE 1 CERTIFICATE OF CORRECTION Patent No. 3,129,384 April 14, 1964 Max J, Kramer rror appears in the above numbered pat- It is hereby certified that e the said Letters Patent should read as ent requiring correction and that corrected below.

Column 1, line 35 for "but" read bus Signed and sealed this 28th day of July 1964b (SEAL) Attest:

EDWARD J. BRENNER Commissioner of Patents ESTON G. JOHNSON Attesting Officer 

1. AN AMMETER FOR MEASURING ANODE BAR CURRENT OF ELECTROLYTIC REDUCTION CELLS AND THE LIKE COMPRISING, (A) A MOUNTING PLATE PROVIDED WITH A PROBE IN THE FORM OF A TUBE PROJECTING FROM ITS REAR FACE AND WITH A COVER BOX ON ITS FRONT FACE AND WITH AN ELONGATED HANDLE EXTENDING FORWARDLY OF ITS FRONT FACE FOR PLACEMENT OF THE OUTER END OF THE PROBE AGAINST A VERTICAL ANODE BAR WITH THE PLATE IN UPRIGHT POSITION, BOTH PLATE AND TUBE BEING OF NON-MAGNETIC MATERIAL, (B) A SHAFTED CYLINDRICAL PERMANENT MAGNET ROTOR POLARIZED ACROSS A DIAMETER AND COAXIALLY ROTATABLY MOUNTED WITHIN SAID TUBE IN PROXIMITY TO THE OUTER END THEREOF AND HAVING ITS SHAFT EXTENDING INTO SAID COVER BOX, (C) A RADIAL POINTER ON SAID SHAFT ADAPTED TO COOPERATE WITH A GRADUATED SCALE ON SAID PLATE, AT LEAST A PORTION OF SAID BOX BEING TRANSPARENT FOR SCALE VISIBILITY, (D) A PAIR OF ESSENTIALLY SEMI-CIRCULAR, MAGNETIC STATOR POLES OF UNIFORM THICKNESS SECURED TO THE INNER WALL OF SAID TUBE IN ENCIRCLING RELATION TO SAID ROTOR AND SEPARATED BY NARROW AIR GAPS AT THEIR OPPOSED PERIPHERAL EDGES, WHEREBY THE ROTOR IS MAGNETICALLY HELD AND RESTRAINED TO REMAIN IN ZERO POINTER SCALE INDICATING POSITION BY ITS OWN FLUX WITH ITS FLUX LINE IN ALIGNMENT WITH THE CENTER OF SAID AIR GAPS, (E) SAID STATOR POLES HAVING A FIXED POSITION WITH RESPECT TO A TRANSVERSE PLANE ACROSS SAID PLATE WHICH DISPOSES SAID AIR GAPS IN PERPENDICULAR RELATION TO AND THE CENTER LINE OF SAID POLES IN PARALLELISM WITH SAID PLANE FOR PROVIDING A SUBSTANTIALLY CORRESPONDING RELATIONSHIP OF SAID ROTOR AND STATOR POLES WITH RESPECT TO THE ANNULAR MAGNETIC FIELD AROUND THE ANODE BAR WHEN THE AMMETER IS IN CURRENT MEASURING POSITION AGAINST THE BAR, (F) AND A RETURN SPRING SECURED BETWEEN SAID SHAFT AND SAID PLATE FOR LIGHTLY BIASING SAID ROTOR TO RETURN TO ITS ZERO POSITION AND PRODUCING SCALE LINEARITY BY COMPENSATING FOR LOSS OF AIR-GAP RESTRAINT WITH INCREASING CURRENT, (G) SAID STATOR POLES HAVING A THICKNESS OF ABOUT 1/16 INCH FOR MINIMUM MASS AND TO REMAIN MAGNETICALLY UNSATURATED SO AS TO CONTINUOUSLY EFFECTIVELY SHIELD SAID ROTOR FROM STRAY MAGNETIC FIELD LINES INHERENTLY PRESENT AT SAID CELL, AND THE WIDTH OF SAID AIR GAPS BEING LESS THAN 3/32 INCH TO MINIMIZE STRAY FIELD LINES PASSING THERETHROUGH TO THE ROTOR. 